The company I work for is a sub supplier to many automotive OEM's. To measure their satisfaction, we send out a survey and create a customer satisfaction index. Alot of them don't return the survey and don't allow their employees to fill it out either.
Outside of a survey, what other tools/best practices can be used to measure customer satisfaction?

I would recommend that you can use mystery shopping activities to get your customers to feedback their experience. I would also recommend maybe using focus groups or even simple call to get your customers to feedback what they thought about your services.

Also marketing and communicating your compliments, comments and complaints processes would be beneficial to capture your customers' feedback.

One key reason people do not return the survey back is that some surveys are too long to complete. Have you tried to cut it down, or maybe just using the Net Promoter Score technique to get the information you would like?